1. Field of the Invention
This invention relates to raw materials for producing ruthenium or ruthenium compound thin films by CVD.
2. Description of the Related Art
In recent years, ruthenium or ruthenium compounds have been being applied as materials for thin film electrodes used in semiconductor devices such as DRAM and FERAM. This is because these materials are extremely excellent in electric properties such as specific resistance, and, for example in DRAM, the use of the materials for the storage electrodes of its capacitors have been examined and is considered to be able to contribute largely to higher density DRAM. And ruthenium or ruthenium compound thin films have been expected to become one of the key materials for thin film electrodes and attracted considerable attention.
As the method for producing ruthenium or ruthenium compound thin films, not only sputtering, but also chemical vapor deposition (hereinafter referred to as CVD) is often used. The reason that CVD is often used is that it makes it easier to produce uniform thin films, in addition, it is superior to sputtering in step coverage.
Ruthenium compounds which have been expected to be materials for use in production of ruthenium or ruthenium compound thin films by CVD and the use of which has been examined include, for example, bis(ethylcyclopentadienyl)ruthenium having the formula shown below. This organic ruthenium compound is obtained by replacing hydrogen atoms of the two cyclopentadiene rings bis(cyclopentadienyl)ruthenium by ethyl groups. 
The compound, bis(ethylcyclopentadienyl)ruthenium, has been considered to be qualified as a CVD raw material because it is excellent in handling properties, due to its low melting point and the liquid state at room temperature, and in production efficiency, due to its high vapor pressure.
Further, in recent years, from the viewpoint of reduction of thin film production costs and effective use of resources, introduction of recycling techniques has also been examined for CVD raw materials. In such recycling, it is required to efficiently separate/purify unreacted compounds from used materials. In bis(ethylcyclopentadienyl)ruthenium, the unreacted compound is relatively easy to separate and refine, due to its high vapor pressure and good thermal stability, and its effective recycle is made possible by proper means such as distillation.
In thin film electrodes, it goes without saying that their purity and morphology are important, but on the other hand their adhesion to a substrate is also important. As a substrate for semiconductor devices, SiO2 or a SiO2 film is often used, a ruthenium thin film produced from bis(ethylcyclopentadienyl)ruthenium, however, has a problem of relatively low adhesion to SiO2. And to cope with this problem, a method has been employed in which a Ru front-end is formed by sputtering before forming a thin film by CVD. However, this leads to increase in the number of processes. Thus, there have been demands for development of organic compounds which, as CVD raw materials, permit the production of thin films having higher adhesion properties.
To meet the demand for higher adhesion properties, organic ruthenium compounds having been proposed in recent years are tris(xcex2-diketonato)ruthenium having the following formula (for further details of this compound, refer to Japanese Patent Laid-Open No. 2000-212744). 
wherein R and Rxe2x80x2 represent different alkyl groups with 1 to 4 carbon atoms, respectively.
The above organic ruthenium compounds consist of ruthenium and three xcex2-diketons coordinated therewith, and their states at room temperature differ depending on their substituents, in other words, there are tris(xcex2-diketonato)ruthenium in the liquid state and those in the solid states. Since the compounds being in the solid state at room temperature are highly soluble in organic solvents, as long as they are dissolved in an organic solvent, they can be used as CVD raw materials. Thus, both liquid and solid tris(xcex2-diketonato)ruthenium are said to be possibly used as CVD materials. Since the ruthenium thin films produced from tris(xcex2-diketonato)ruthenium excel in adhesion to SiO2, much hope is placed on these compounds, in this respect.
However, according to the inventors of this invention, in tris(xcex2-diketonato)ruthenium which are in the liquid state at room temperature (such as tris(2,4-octadionato)ruthenium wherein R1 is a methyl group and R2 is a butyl group and tris(6-methyl-2,4-heptadionato)ruthenium wherein R1 is a methyl group and R2 is an isobutyl group), their thin film production efficiency is low because of their low vapor pressure, and to ensure a certain efficiency, the heating temperature must be increased at the time of thin film production. At the time of heating, if the vaporization temperature is increased higher than necessary and the raw materials are excessively heated, the compounds tend to decompose even in an inert atmosphere. Therefore, when producing their thin films using a large volume of raw material gas, it is difficult to control the heating of the raw materials. Thus, liquid xcex2-diketonatoruthenium are unfit for thin film production on large-size substrate and mass production.
Further, if the vapor pressure is low, it is difficult to separate/purify the unreacted compounds from the used raw materials by distillation etc.; and moreover, in the above easy-to-decompose compounds, the amount of the unreacted compounds remaining in the used raw materials is very small, and even if they can be separated, it is impossible to recover them efficiently. Thus the use of these compounds for forming thin films results in higher production costs.
As to the solid tris(xcex2-diketonato)ruthenium, taking into consideration their recycling, they are not necessarily preferable, even if they can be used as CVD raw materials once they are dissolved in solvents. The reason is that it is often difficult to recover solid matters by distillation once they are dissolved in liquids.
Thus, though tris(xcex2-diketonato)ruthenium have the advantage of providing thin films with good adhesion, they still have problems of being low in thin film production efficiency due to their low vapor pressure and in possibility of their recycling.
This invention has been made in light of the above described background. Accordingly, an object of this invention is to provide, on the assumption that the raw material compounds for use in production of ruthenium thin films or ruthenium compound thin films by CVD are in the liquid state at room temperature, organic ruthenium compounds which have the advantages the above described compounds have, that is, the advantages of having good thermal stability and high vapor pressure, and hence being fit for recycling and of being able to provide thin films with high adhesion to a substrate. Another object of this invention is to provide a method for producing thin films using the above organic ruthenium compounds.
To solve the above described problems, the inventors started with the analysis of the course of the formation of thin films in CVD and presumed the constitution of preferable compounds. In CVD, vaporized raw material compounds are carried on a substrate, and decomposed and oxidized on the same so that the object materials as constituents of thin films are deposited as thin films.
In the above described CVD, the process of ruthenium thin film formation consists mainly of two steps. The first step is a step of forming ruthenium crystal nucleuses on a substrate, which is also referred to as latent period or incubation period. In this step, there exists no thin film, but countless crystal nucleuses are dotted on the substrate and grow in the planar direction.
The substantial growth of ruthenium crystal as a thin film occurs in the step next to the first step (the latent period). In the step, the decomposition reaction of the raw material gas is accelerated by the catalytic action of the crystal nucleuses produced in the first step and thereby the rapid growth of the thin film is caused.
The inventors considered that the adhesion of thin films to a substrate depends largely on the number of crystal nucleuses produced in the first step and the thin films"" affinity for the materials which consist of the substrate. In other words, the inventors considered that to produce thin films having good adhesion to a substrate, it is desirable to use, as raw materials, compounds capable of producing more crystal nucleuses having affinity for the substrate materials.
Then, the inventors examined tris(xcex2-diketonato)ruthenium for the factors that permitted thin films of good adhesion to be formed. As a result, they found that Ruxe2x80x94O bonds contained in the molecule largely contributed to the adhesion of thin films to a substrate. According to the inventors, due to the Ruxe2x80x94O bonds, the crystal nucleuses produced on the substrate in the first step are deposited in the form similar to ruthenium oxide whose oxygen content is high, and the high adhesion properties of the crystal nucleuses similar to ruthenium oxide serve to improve the adhesion properties of thin films.
Based on the above mentioned consideration, the inventors concluded that to ensure the adhesion properties of thin films, organic compounds containing oxygen in their molecules are preferably used.
On the other hand, consideration of vapor pressure, which is the other problem to be solved by this invention, revealed that vapor pressure tended to be low in compounds having high molecular weights. And it is considered that, since many of the conventional tris(xcex2-diketonato)ruthenium have three ligands (xcex2-diketones), which have much higher molecular weights compared with the ligands of bis(ethylcyclopentadienyl)ruthenium, coordinated with ruthenium, the molecular weights of the ruthenium compounds become higher, which causes low vapor pressure.
Thus, based on the above mentioned consideration, the inventors concluded that preferable organic ruthenium compounds had xcex2-diketones, which are ligands containing oxygen, coordinated with ruthenium and had low molecular weights. And compounds were examined which met the above requirement and the preconditions that they were in the liquid state at room temperature. As a result, they found the following three types of organic ruthenium compounds which have two xcex2-diketones and specified ligands coordinated with ruthenium, and they finally thought of this invention.
A first type of organic metal compounds of this invention are raw material compounds for use in CVD which contain organic ruthenium compounds as a main ingredient, the organic ruthenium compounds having two xcex2-diketones and one diene coordinated with ruthenium as shown by the following structural formula. 
A second type of organic metal compounds of this invention are raw material compounds for use in CVD which contain organic ruthenium compounds as a main ingredient, the organic ruthenium compounds having two xcex2-diketones and one diamine coordinated with ruthenium as shown by the following structural formula. 
A third type of organic metal compounds of this invention are raw material compounds for use in CVD which contain organic ruthenium compounds as a main ingredient, the organic ruthenium compounds having two xcex2-diketones and two organic ligands coordinated with ruthenium as shown by the following structural formula. 
As described above, the three types of organic ruthenium compounds of this invention have a common characteristic of having two diketones, as a factor that improves the adhesion of their thin films, coordinated therewith, and they further have specified ligands, as a factor that ensures high vapor pressure and handling properties (possibility of keeping themselves in the liquid state) required for CVD raw materials, coordinated therewith.
The vapor pressures of the organic ruthenium compounds of this invention tend to be low as the molecular weights of the xcex2-diketones coordinated with ruthenium of the organic ruthenium compounds of this invention become high, as aforementioned; therefore, in any of the organic ruthenium compounds of this invention, the xcex2-diketones preferably have low molecular weights. Thus, in this invention, the substituents R1, R2 of the xcex2-diketones are alkyl groups, respectively, and the total number of their carbon atoms is restricted within a prescribed range. Specifically, in the first type of ruthenium compounds, the total number of the carbon atoms of R1 and R2 is 3 to 5. The reason for this is that if the number of the carbon atoms is less than 3, the compounds are hard to keep in the liquid state at room temperature, whereas if the number of the carbon atoms is more than 5, the molecular weights of the ruthenium compounds become high and their vapor pressures become low. And for the same reason, in the second and third types of ruthenium compounds, the total number of the carbon atoms of R1 and R2 is 2 to 5.
Preferably, the substituents R1 and R2 of the xcex2-diketones are different alkyl groups, in other words, the xcex2-diketones are of asymmetrical shape. This is because the compounds which have the xcex2-diketones of asymmetrical shape coordinated with ruthenium are easier to keep in the liquid state.
Taking into account the above conditions, the concrete examples of the xcex2-diketones preferably used in this invention include, for example, 2,4-hexanedione, 5-methyl-2,4-hexanedione, 2,4-heptanedione, 5-methyl-2,4-heptanedione, 6-methyl-2,4-heptanedione and 2,4-octanedione. Of these xcex2-diketones, preferably those having branched structure are selected for the ruthenium compounds, compared with those having straight-chain structure, because the ruthenium compounds having the xcex2-diketones of branched structure coordinated therewith have higher vapor pressures. Accordingly, of the above examples, 5-methyl-2,4-hexanedione, 5-methyl-2,4-heptanedione and 6-methyl-2,4-heptanedione are particularly preferable.
At the same time, the organic ruthenium compounds of this invention have one diene or diamine coordinated with ruthenium. Selecting these ligands permits the organic ruthenium compounds of this invention to have high vapor pressures, and using such ruthenium compounds as CVD raw materials enables efficient thin film production.
The dienes coordinated with ruthenium of the first type of organic compounds are not limited to chain dienes, but may be cyclic dienes. Although the dienes have two double bonds per molecule, the relationship between the distance from one double bond to the other and the distance of d orbital of ruthenium affects the thermal stability of the compounds. According to the inventors, if the distance from one double bond to the other is too short, the coordination of the diene does not occur, whereas if the distance is too long, the thermal stability of the compounds is lowered even if the coordination occur. Preferably the molecular weights of the coordinated dienes are relatively low. Just like the case of xcex2-diketones, the dienes having low molecular weights permit ruthenium compounds to have high vapor pressures. Taking into account these conditions, preferable coordinated dienes include, for example, 1,5-cyclooctadinene, norbornadiene and 1,4-cyclohexadiene.
Then, as to the diamines which are ligands of the second type of organic compounds, one of the requirement for the ligands is that they have low molecular weights. And, similar to the first type of compounds, the thermal stability of the second type of organic compounds depends on the relationship between the atomic distance between the two nitrogen atoms and the distance of d orbital of ruthenium. Taking this into account, the substituents R6 to R8 of the diamines are selected from hydrogen and alkyl groups and the total number of the carbon atoms is 2 to 8. And preferable coordinated diamines in this second type of compounds include, for example, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine.
In this patent application, are disclosed, as the third type of compounds, the organic ruthenium compounds which have two organic ligands coordinated with ruthenium. The requirements for the organic ligands are that they are what are called monodentates, they have low molecular weight, and that their bond strength to ruthenium is high. In particular, preferable organic ligands include, for example, olefins, amines, nitrites and carbonyls. As concrete examples of each organic ligand, the olefins include, for example, ethylene, propylene, 2-methylpropylene, butene, 2,4-butadiene. The amines include, for example, trimethylamine and triethylamine. And the nitrites include, for example, acetonitrile and acrylonitrile.
Then, the methods for producing the organic ruthenium compounds of this invention will be described. The organic ruthenium compounds of this invention consist of ruthenium and two types of organic ligands coordinated therewith, and there are two methods for producing the same.
A first production method includes the steps of: reacting a ruthenium compound with a diene compound, diamine compound or the like to produce a ruthenium diene compound etc. as a precursor; and reacting the precursor with a xcex2-diketone compound to produce an object compound. This method is suitable for producing the first type of compounds (the compounds having a diene coordinated with ruthenium) of this invention. For example, in the method for producing bis(5-methyl-2,4-hexanedionato)(norbornadiene)ruthenium (R1=methyl group, R2=propyl group), which is one of the first type of organic ruthenium compounds of this invention, first a ruthenium compound (ruthenium chloride etc.) is reacted with norbornadiene in an organic solvent to produce norbornadieneruthenium chloride as a precursor. Then the norbomadieneruthenium chloride is reacted with 5-methyl-2,4-hexanedione to produce bis(5-methyl-2,4-hexanedionato)(norbornadiene)ruthenium.
A second method includes the steps of: reacting a ruthenium compound with a xcex2-diketone compound to produce tris(xcex2-diketonato)ruthenium as a precursor; and reacting the precursor with a diene compound, diamine compound or the like to produce an object compound. This method is suitable for producing the second type of compounds (the compounds having a diamine coordinated with ruthenium) and the third type of compounds (the compounds having two organic ligands coordinated with ruthenium) of this invention. For example, in the method for producing bis(5-methyl-2,4-hexanedionato)(N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine)ruthenium (R1=methyl group, R2=propyl group, R6 to R11=methyl groups), which is one of the second type of organic ruthenium compounds of this invention, first a ruthenium compound (ruthenium chloride etc.) is reacted with 5-methyl-2,4-hexanedione in water to produce tris(5-methyl-2,4-hexanedionato)ruthenium as a precursor. Then the precursor is reacted with N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine to produce bis(5-methyl-2,4-hexanedionato)(N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylenediamine)ruthenium.
Lastly, a method for forming thin films which applying CVD raw materials containing the organic ruthenium compounds of this invention as a main ingredient. The method is basically the same as the general type of CVD. Specifically, the method includes the steps of: vaporizing a raw material compound; carrying the vaporized raw material compound on a substrate; and decomposing the raw material to deposit ruthenium or a ruthenium compound on the substrate.
As to the vaporization of the raw material compounds, since the organic ruthenium compounds of this invention can be in the liquid state at room temperature, it may be performed by directly heating the organic ruthenium compounds or by dissolving the organic ruthenium compounds of this invention in proper solvents and heating the solution for vaporization. In the latter case, as a solvent are used organic solvents such as methanol, ethanol and propanol.
As to the system of vaporizing the raw materials, there are two types: one in which the raw materials stored in a raw material container is bubbled while heated; and the other in which the raw materials stored in a raw material container is heat vaporized with a vaporizer. Any of the above types are applicable to the compounds of this invention. However, to produce thin films in a more stable manner, the system using a vaporizer is preferable. As to the heating temperature at the time of raw material vaporization, preferably it is 80 to 150xc2x0 C. when heating the raw materials directly.
The methods for decomposing the raw material molecules having been vaporized and carried on the substrate surface are not limited to any specific ones, and any methods such as thermal CVD and plasma enhanced CVD can be employed. The thermal CVD is particularly preferable, because the system is simple and there is no concern about damage to the substrate since the decomposition temperature of the raw materials of this invention is relatively low. Preferably the ruthenium compounds are decomposed at substrate temperatures of 250xc2x0 C. to 400xc2x0 C.
In this CVD process, preferably a reduced pressure atmosphere is maintained inside the reactor. Reducing the pressure inside the reactor provides satisfactorily uniform film distribution and good step coverage. The preferable range of the pressure inside the reactor is 10 to 500 Pa.